1. Field of the Invention
The present invention relates to a circuit for detecting overcurrent state of main current flowing in a power transistor, and further to a semiconductor circuit for protecting the power transistor itself from the overcurrent on the basis of the detection of overcurrent state and also controlling driving of another power transistors. The technology of the present invention is applicable to an inverter circuit for a motor, for example.
2. Description of the Background Art
(Background Art 1)
A conventional overcurrent protection circuit for an IGBT, one of the power transistors, will now be described referring to FIG. 11, and FIG. 2A to FIG. 12F.
FIG. 11 is a block diagram showing an IGBT driving circuit device including a conventional IGBT overcurrent protection circuit. Note that the circuit shown in FIG. 11 relates to technical know-how or inside technology of the applicant of the present invention, which is publicly not known.
In FIG. 11, the character 1P denotes an IGBT as a power transistor (whose collector is connected to an inductance and a free-wheel diode not shown), 2P denotes a current detecting circuit, 3P denotes an overcurrent detecting circuit (a comparison circuit), 4P denotes an overcurrent decision circuit (an AND circuit), 5P denotes an error output terminal, 6P denotes an input terminal, 7P denotes an IGBT driving circuit, and 8P denotes a gate resistance.
This circuit is characterized in the following points: (1) one input end of the overcurrent decision circuit 4P is connected to the input signal line 15P which is connected to the input terminal 6P at the node N1P; and (2) the output signal line 13P which branches from the output node N2P of the circuit 4P is connected to one input end of the NOR circuit in the IGBT driving circuit 7P.
Now, if a signal at an "H" level is inputted to the input terminal 6P, the IGBT driving circuit 7P outputs a signal at an "H" level, which causes the gate of the IGBT 1P to go to an "H" level through the gate resistance 8P to turn on the IGBT 1P. If a signal at an "L" level is inputted to the input terminal 6P in this condition, the IGBT driving circuit 7P outputs a signal at an "L" level, which places the gate of the IGBT 1P on an "L" level through the gate resistance 8P to turn off the IGBT 1P. This change in state is shown in the timing chart in FIG. 12A to FIG. 12F.
As shown in FIG. 12A to FIG. 12F, there occurs an ON delay time OND between when the input signal at the "H" level corresponding to an ON signal level is applied to the input terminal 6P and when the IGBT 1P changes from the OFF state to the ON state. Similarly, there occurs an OFF delay time OFD between when the input signal at the "L" level corresponding to an OFF signal level is applied to the input terminal 6P and when the IGBT 1P changes from the ON state to the OFF state. These delay times OND and OFD occur due to the IGBT driving circuit 7P.
When the "H" level input signal is applied to the input terminal 6P, the IGBT 1 turns on after an elapse of the ON delay time OND, and the current flowing in the IGBT 1P at this time is monitored by the current detecting circuit 2P. If the overcurrent detecting circuit 3P detects that the current flowing in the IGBT 1P has reached an overcurrent state, the overcurrent decision circuit 4P outputs a signal at an "H" level only when the input signal is the ON signal at that time, so as to control the output of the IGBT driving circuit 7P to the "L" level, thereby disconnecting the gate of the IGBT 1P to bring the IGBT 1P in the OFF state. At the same time, the circuit 4P externally signals that the IGBT 1P is in the overcurrent state from the error output terminal 5P. FIG. 12 to FIG. 12F show the error output state as an overcurrent state which takes place and is detected when the third ON signal is inputted.
Since the structure of the current detecting circuit 2P uses resistance herein, the overcurrent detecting circuit 3P detects the overcurrent state when the voltage across both ends of the resistance becomes larger than a threshold voltage set in the overcurrent detecting circuit 3P. The value of the resistance is set to generate as small voltage as possible, since large voltage generated at the resistance causes large power loss. However, if noise gets on the resistance in the current detecting circuit 2P when the IGBT 1P is in the OFF state, and if the noise is equal to or larger than the voltage set in the overcurrent detecting circuit 3P, the overcurrent detecting circuit 3P will detect the noise as overcurrent. To prevent this problem, such erroneous determination of detecting an overcurrent state due to a noise when the operation of the IGBT is OFF can be prevented if it is structured to detect the overcurrent only when an input signal at the ON signal level is inputted to the input terminal. Accordingly, in the circuit shown in FIG. 11, the node N1P and one input end of the circuit 4P are connected through the signal line 15P, so that the overcurrent decision circuit 4P determines that the IGBT 1P is in an overcurrent state when the overcurrent detector 3P detects an overcurrent (the "H" level output) with the ON signal level ("H" level) input signal applied to the input terminal 6P.
(Second Background Art; Prior References)
Overcurrent protection circuits for power transistors disclosed in preceding references include those shown in (1) Japanese Patent Laying-Open No. 7-183781, (2) Japanese Patent Laying-Open No. 6-276073, and (3) Japanese Patent Laying-Open No. 6-105448.
In the reference (1), a current detecting resistance detects current flowing in the IGBT as a voltage value. When an overcurrent state is detected, a control thyristor is turned on with that voltage to generate a turning-off instruction to the IGBT.
In the reference (2), it is detected whether current flowing in the IGBT is placed in an overcurrent state due to a short-circuit trouble on the basis of the current flowing in the IGBT and part of input signal to an IGBT driving circuit. This function is equivalent to that of the above-described circuit shown in FIG. 11. However, the reference (2) aims principally at protecting the IGBT from overcurrent flowing due to an accident when the IGBT is in the ON state.
In the reference (3), the overcurrent state is detected only by detecting the current flowing in the IGBT, on the basis of which the driving voltage for the IGBT is controlled. The reference (3), too, focuses on detection of overcurrent caused by a short-circuit trouble in the ON state.
The overcurrent protection circuit shown in FIG. 11 raises a new problem caused by the presence of the signal line 15P. This will be described below referring to FIG. 11 and the timing chart shown in FIG. 13A to FIG. 13G.
Now, suppose that the input signal changed from the "H" level corresponding to the ON signal level to the "L" level corresponding to the OFF signal level, and then the current flowing in the IGBT 1P reached the overcurrent state before an elapse of the OFF delay time OFD required for the IGBT 1P to change from the ON state to the OFF state (at time T1). In this case, the current detecting circuit 2P outputs a voltage indicating that an overcurrent is flowing in the IGBT 1P to the input end of the overcurrent detecting circuit 3P, which causes the circuit 3P to output an output signal at the "H" level. However, since the input signal is at the "L" level corresponding to the OFF signal level at this time, the overcurrent decision circuit 4P does not recognize the current flowing in the IGBT 1P to be overcurrent, and it therefore cannot signal a decision indicating the overcurrent state of the IGBT 1P, or an error output, to the outside. As a result, the supply of the input signal is kept uninterrupted after time T1. When the input signal changes to the ON signal level again, the overcurrent decision circuit 4P cannot signal the occurrence of the overcurrent state to the outside until the IGBT 1P turns on and the overcurrent flows again, that is to say, until time T2. The input signal supplied from outside is then fixed at the OFF signal level after that. The IGBT 1P is thus turned off at time T2 at which the main current is presenting a still larger current value, which inevitably causes a large surge voltage. Furthermore, another IGBTs standing in the ON state, not shown, are turned off at that timing, too.
This problem also occurs in the above-mentioned prior references (1) to (3), but the references (1) to (3) do not suggest this problem. Moreover, in the reference (1), the problem that an overcurrent state is erroneously detected due to a noise and is externally outputted when the IGBT is OFF is left unsolved. Accordingly, the references (1) to (3) do not provide means for solving this problem.
As have been described with the example shown in FIG. 11, FIG. 12A to FIG. 12F, FIG. 13A to FIG. 13G, the conventional overcurrent protection circuit externally signals that the power transistor is in an overcurrent state from the error output terminal when the overcurrent detector (resistance) detects an overcurrent state with the input signal presenting an ON signal. Therefore, if the overcurrent state occurs before the power transistor makes a transition from ON operation to OFF operation, the overcurrent state cannot be detected at the moment of its occurrence. It is when the ON delay time has passed after the next input of the input signal at the ON signal level, that is, when the power transistor has reached the overcurrent state again, that the overcurrent state is detected and the result is signaled to the outside. The current value flowing in the power transistor increases in this delay in the detecting timing, which causes the problem that the power transistor is controlled to the turned-off state in the condition or timing where the value of the main current has grown to a still larger current value over the overcurrent detection level (a reference level).
This problem may occur not only when the load includes only an inductance component of a load device such as a motor but also when the load includes resistance. It can be said that this problem generally occurs when an overcurrent takes place in a switching circuit using a power transistor.